HI Folks,

I’m an amateur photographer in need of an add-on to my home-made flash gun sound trigger.

The current unit works perfectly for photographing things like balloon bursts etc. The balloon makes a bang, dynamic mic creates signal which feeds a preamp. The preamp output connects to an SCR which effectively creates a closed circuit across the low voltage feed from the flash and the flash fires. Great!

My problem is that when shooting an event involving things like air-rifles, light bulbs, glasses etc things get a bit noisy which means there are multiple signals created by the mic etc which causes the flash to fire more than once, ruining the exposure.

As I’m an electrician by trade it would be simple to use relays and a latching circuit, but relays are way too slow, I need a solid-state solution. Unfortunately, as an electrician my legs go wobbly at the very mention of semiconductors. I’ve basically copied the info from YouTube

Is there a simple set of extra components that I can add to force the unit to switch off the signals after the original sound event?
Images below represent what I've got working so far.
Thanks for reading.
Eugene.

 

One solution is what's called a one shot timer (a.k.a. monostable multivibrator). Here's a thread about it:

https://forum.allaboutcircuits.com/threads/next-submission-555-monostable.14018/

An excerpt:

"This particular monostable multivibrator (also known as a monostable or timer) is not a retriggerable type. This means once triggered it will ignorefurther inputs during a timing cycle, with one exception, which will be discussed in the next paragraph. "

 

Images below represent what I've got working so far

The circuit in the blue enclosure, is that the actual product you are using? I have seen flash circuits using the 555 chip that look similar to the photo.
SG

 

Do you have a schematic for the audio source trigger circuit?

 

Post deleted

 

Do you have a schematic for the audio source trigger circuit?

Hi Crutchow, Many thanks for your response to my question. The preamp is a kit from Vellman, a shematic is shown in the pdf here

 

The circuit in the blue enclosure, is that the actual product you are using? I have seen flash circuits using the 555 chip that look similar to the photo.
SG

Hi sghioto, Many thanks for reponding. The blue enclosure is a rig I made myself to contain the vellman preamp kit (shown here) and all the connections to the rest of the gear. No idea what the chip is or what it does.
Thanks again.

 

One solution is what's called a one shot timer (a.k.a. monostable multivibrator). Here's a thread about it:

https://forum.allaboutcircuits.com/threads/next-submission-555-monostable.14018/

An excerpt:

"This particular monostable multivibrator (also known as a monostable or timer) is not a retriggerable type. This means once triggered it will ignorefurther inputs during a timing cycle, with one exception, which will be discussed in the next paragraph. "

Hi MrSoftware,
Many thanks for the info. Unfortunately on reading the article I round almost all of it went over my head. I have however found a kit from RK Education that may help me. Is this the sort of thing you were refering to? If so, could I use my existing 12V battery pack to power it or must it be 9V. Also, there is the question of how do I integrate it into my existing rig?
Huge thanks for you patience and support.

 

No idea what the chip is or what it does.

The chip is the amplifier for the microphone. Adding a NE555 one shot circuit looks doable using the RK module.
SG

 

Here's how I would connect the RK Education circuit to the Vellman audio amp.
SG

 

Hi MrSoftware,
Many thanks for the info. Unfortunately on reading the article I round almost all of it went over my head. I have however found a kit from RK Education that may help me. Is this the sort of thing you were refering to? If so, could I use my existing 12V battery pack to power it or must it be 9V. Also, there is the question of how do I integrate it into my existing rig?
Huge thanks for you patience and support.

At first glance that looks like the right idea to me. Read the docs and see if it will trigger once and then have an adjustable delay until it can trigger again. Will it be a problem if the signal stays high or low (whichever triggers your flash) for a full second or two?

 

From how the problem is stated in post #1, I think a latch circuit is needed rather than a monostable providing a fixed pulse width.
Here's a '555-based approach I'd try :

This uses the open-collector discharge pin of the '555, in case the flash unit needs a volts-free trigger input.
The SCR is not required, nor is any modification of the Velleman preamp output circuit.

 

This mod to the Vellman amp may work depending on the voltage level coming out of the Flash shoe. When the SCR is triggered it will drop the voltage on pin 3,5 of the amp through the diode and discharge C2. This will shut the amp down until C2 recharges which takes about 2 seconds using a 220uf cap.
SG

 

Dear All,
Huge thanks for your help thus far. It really is most encouraging and very appreciated. To answer questions I think I've been asked:

• The speedlight/flashgun has an output voltage of approx 5V. All that is needed to fire the flash is something/anything to make closed circuit the two wires in the cable from the flash (a N/O switch).
• For my purposes, the delay before any mod can reset needs to be at least 1 second as that is typically how long the shutter is open for (work in the dark, flash creates exposure, shutter speed largely irrelevant).
• I may well try more than one of your suggested solutions as electronic components seem really cheap. Feel another hobby coming along
• My wife has plans for me tomorrow (trust me there is no such word as retired) so if I don't respond further for a day or so it's because I'm too busy with grandchildren

 

How about a different approach: Only charge the flash capacitor once, and then switch off the charger just before you anticipate the trigger to happen. When the trigger for the exposure that you want happens, the flash storage capacitor will be discharged and no matter how many triggers arrive no flash will happen. The only obvious flaw is if you need to be doing something else for a few minutes prior to the flash being triggered. The advantage is that it is a really simple way to avoid second flashes. Just replacing the power switch for the flash circuit, but not for the trigger ciecuit, with a momentary NO pushbutton.

 

How about a different approach: Only charge the flash capacitor once, and then switch off the charger just before you anticipate the trigger to happen.

This would work with older speedlites, but the one pictured is a Yongnuo YN560 which will not fire if it is not turned on.

 

OK, a little experimentation with a YN600EX-RT and a 555 one shot confirms that is it trivially possible to trigger the flash and hold it disabled for the interval of the output. Rather than trigger once and disable the trigger circuit until it is reset or a timer has elapsed, you can simply hold the trigger on until the timer has lapsed which is simpler.

I used a 2N2222 as the trigger, but I don't know if that's what I would use in a finished version.

Some observations:

The flash is extremely sensitive to triggers and can respond at a very high rate. The trigger must be decisive or there will be false flashes.
The flash doesn't recycle until the trigger is released.

NOTE: I triggered the flash from the shoe and not the PC cord socket since I didn't have one handy but I don't think there will be any difference between them.

I suspect the 555 could be connected to the amp output with little trouble and a BJT output seems sufficient to switch the flash.

 

For reference, the circuit I used is the same as this one:

https://www.allaboutcircuits.com/tools/555-timer-monostable-circuit/

With the output through the 2N2222 for triggering the flash to its shoe and center pin.

 

The speedlight/flashgun has an output voltage of approx 5V.

I would try the Vellman amp mod I posted in #13. You won't need a 555.
SG

 

I would try the Vellman amp mod I posted in #13. You won't need a 555.
SG

The voltage on the PC contacts and hot shoe measures as 3V.